This invention relates to tape drive system for reading and writing data on a data storage medium and a system for detecting the physical edge of the data storage medium as it moves under the read/write heads in the data storage subsystem.
It is a problem in the field of tape drive system to detect the physical edge of the data storage medium as it is in motion. This is especially problematic in the field of removable data storage media, such as magnetic tape cartridges that contain magnetic tape media, where the data is recorded on the data storage medium in a horizontal direction along the length of the data storage medium. This format is typically termed xe2x80x9clinear recordingxe2x80x9d and comprises a series of closely spaced tracks written in parallel across the width of the data storage medium. In order to maximize the data storage capacity of the data storage medium, the tracks are made as narrow as possible and positioned closely together. It is therefore necessary to accurately position the tracks on the data storage medium to avoid crowding of the tracks, to avoid wasting the data storage space provided by the width of the data storage medium, and the accurately select tracks to read data previously written on these tracks. It is therefore necessary to find an accurate reference point on the data storage medium as a starting point for the first track that is recorded. The edge of the data storage medium is typically used by tape drive system for this reference point.
Individual data tracks are very narrow and a large number of tracks can therefore be written on the data storage medium on a given width of magnetic tape. In a magnetic tape data storage medium, magneto-resistive read/thin film write heads are used to read and record the data. A plurality of magneto-resistive read/thin film write heads and their associated electronics are typically grouped into a single physical read/write head apparatus so that a plurality of tracks can be concurrently written and read. This read/write head apparatus is mounted on a movable carriage mechanism that functions to position the magneto-resistive read/thin film write heads over a selected set of tracks on the magnetic tape. The alignment of the magneto-resistive read/thin film write heads with the tracks, especially tracks previously written on the magnetic tape, requires the use of precision read/write head positioning apparatus and the location of a predefined reference point on the magnetic tape to ensure that the proper set of tracks are being read/written.
There are numerous existing magnetic tape head positioning systems for effecting the alignment of the magneto-resistive read heads with the tracks written on the magnetic tape. These magnetic tape head positioning systems either write alignment tracks on the magnetic tape as a reference point or dynamically determine the edge of the magnetic tape as the reference point.
U.S. Pat. No. 4,476,503 teaches a method of magnetic tape edge detection where a write head is positioned in-line with the magneto-resistive read head. The edge of the magnetic tape is determined in a single pass operation where both the write head and the magneto-resistive read head are moved in synchronization until they are below the edge of the magnetic tape. The write head is turned on and the motion of the magnetic tape is started. Since the magneto-resistive read head is below the edge of the magnetic tape, initially no signal is detected and only an ambient noise signal is produced by the magneto-resistive read head. Once the magnetic tape is in motion, the write head is turned on and a signal of predetermined characteristics is applied to the write head to produce magnetic flux transitions on the magnetic tape. Both the write head and the magneto-resistive read head are slowly moved together to a position on the magnetic tape. As the two sets of heads cross the edge of the magnetic tape, magnetic flux transitions are produced on the magnetic tape by the write head, which is located upstream of the magneto-resistive read head. These magnetic flux transitions are detected by the magneto-resistive read head as the magnetic tape passes under the magneto-resistive read head and compared to a reference signal indicative of the expected magnetic flux transitions. The detected magnetic flux transitions detected by the magneto-resistive read head are thereby used to define the edge of the magnetic tape.
U.S. Pat. No. 5,111,347 teaches a method of detecting the physical edge of the magnetic tape in a two-step process. In a first step, the write head is positioned to the approximate physical edge of the magnetic tape, as determined by the magnetic tape guide apparatus, and a data signal is written on the magnetic tape. The width of the write head is greater than the uncertainty in the location of the edge of the magnetic tape as determined by the magnetic tape guide apparatus, so the data signal is written partially on the magnetic tape and partially off the edge of the magnetic tape. In the second step, the magnetic tape is rewound and restarted so the previously written data signal can be detected by the read head, which is positioned at the edge of the magnetic tape. The read head is moved vertically past the edge of the magnetic tape and the signal strength of the previously written data signal is measured to determine the edge of the magnetic tape. The increase in the signal strength as the read head is moved on to the magnetic tape is used to identify the magnetic tape edge.
U.S. Pat. No. 5,457,585 teaches a simplified method of edge detection that detects the edge of the magnetic tape in a single pass and does not require the use of the write head. The magneto-resistive read heads output a noise signal, which increases as the read heads rubs on the magnetic tape, which output signal is termed xe2x80x9cthermal noisexe2x80x9d due to the nature of its generation. The noise level of a magneto-resistive read head traveling across blank magnetic tape is not the same as the noise level of a magneto-resistive read head that is not in contact with the magnetic tape. This difference in noise signal output by the magneto-resistive read head can be used to identify the edge of the magnetic tape. This is accomplished by first positioning the magneto-resistive read head below the magnetic tape and measuring the xe2x80x9coff magnetic tapexe2x80x9d ambient noise level. The magnetic tape is placed in motion and the magneto-resistive read head is moved slowly upward, with the noise output being measured at each position of the magneto-resistive read head as it is stepwise moved on to the magnetic tape. This measured noise level is stored in memory for later use. The magneto-resistive read head is finally positioned so that it is located above the edge of the magnetic tape, completely on the magnetic tape, where an xe2x80x9con magnetic tapexe2x80x9d noise level is measured. The edge of the magnetic tape is then determined by calculating the difference between the off magnetic tape noise level and the on magnetic tape noise level as evidenced by the plurality of measurements stored in memory.
A problem with these existing magnetic tape head positioning systems that use an edge detection process is that they either require the use of the write head and the magneto-resistive read head operating in synchronization, or require the collection of a significant amount of data for processing to calculate the edge of the magnetic tape. In both of these cases, the magneto-resistive read/thin film write head must be repositioned after the edge determination is made, since the magneto-resistive read/thin film write head is positioned entirely on the magnetic tape before the edge detection is completed. The time delay occasioned by these calculations causes a significant amount of magnetic tape to be used for the edge detection process.
The above-described problems are solved and a technical advance achieved by the present system for detecting the edge of a moving data storage medium which provides a simplified magnetic tape edge detection process using only the ambient noise level of the magneto-resistive read head as the baseline.
This system for detecting the edge of a moving data storage medium measures the ambient noise level of the magneto-resistive read head when the magneto-resistive read head is completely off the magnetic tape media. The ambient noise is caused by thermal and resistive noise in the magneto-resistive read head itself and its associated read path electronics. The ambient noise results in an output signal from the magneto-resistive read head, which is indicative of the ambient noise level. The system for detecting the edge of a moving data storage medium sets a signal threshold that is a predetermined increase over the magneto-resistive read head output signal generated by the ambient noise. The system for detecting the edge of a moving data storage medium then moves the magneto-resistive read head on to the magnetic tape media, where an increase in the magneto-resistive read head output signal is generated due to the friction of the moving magnetic tape coming into contact with the magneto-resistive read head, causing an increased noise level. As soon as the resultant increase in the magneto-resistive read head output signal exceeds the signal threshold, it is indicative of the presence of the edge of the magnetic tape and can be immediately used as an accurate measure of the edge of the magnetic tape. Thus, the present system for detecting the edge of a moving data storage medium can immediately determine the edge of the magnetic tape, using a simple threshold.